1. Field of the Invention
The present invention relates to a method for embedding watermark into an image and digital video recorder using said method, and more particularly a method for embedding watermark wherein forgery/alternation of image can be identified and the location of forgery/alternation can be verified by embedding watermark into a digital image in real time. Also, said embedment of watermark can be performed simultaneously with compression.
2. Description of the Related Art
The present invention relates to Korean Patent Application No. 2001-62934 entitled, “NETWORK CAMERA FOR PREVENTING FORGERY OF DIGITAL IMAGE, NETWORK CAMERA SERVER AND DIGITAL VIDEO STORAGE SYSTEM AND METHOD FOR AUTHENTICATING DIGITAL IMAGE OUTPUT THEREFROM” filed by the same applicant of the present application, which is included as a reference material of the present invention.
The above-identified application discloses a method for preventing forgery/alternation of images by embedding robust watermark (RW) and/or fragile watermark (FW) into an image signal in real time, and a network camera, network camera server, and digital video storage device applying said method. In particular, in said application, the digital image signal is compressed separately after embedding RW but before embedding RW.
The present invention is an invention improving the method for embedding robust watermark and fragile watermark of the above application. In particular, the present invention discloses a method for embedding RW and/or FW simultaneously with performing compression, a method for increasing the amount of information being embedded, a method for enhancing the embedment rate to be close to real time, a method for embedding watermark which has interoperability when embedding and detecting watermark at the spatial domain and transformation domain, etc. Also, the present invention discloses a digital video recorder (DVR) applying said method for embedding robust watermark and/or fragile watermark.
Before describing the present invention, the digital video recorder applying the method of the present invention will be described in the following.
The DVR (Digital Video Recorder) as the digital CCTV system is widely being used as a monitoring system of the next generation which will replace monitoring systems of the conventional CCD camera, VCR, TAPE, etc. The analogue monitoring system monitors the environment to be monitored, records the necessary image data on a tape, and searches and stores said tape. However, the DVR can transform image data which was shot into digital signals and store them at a hard disc or DVD-RANM, etc. Thus, it appears that the DVR has a number of advantages over the analogue system. Also, one DVR can record and thus control a number of cameras (for example, 16 cameras) and a number of images (16 cut images).
Meanwhile, the method for embedding and extracting watermark of the present invention mainly uses the characteristics of a frequency transform. Transform which can be used in this regard includes wavelet transform (WT) and discrete cosine transform (DCT), and such transforms are mainly used in removing the redundancy of the spatial information of an image at compressed algorithms.
First, wavelet transform is a transformation method advantageous in extracting the characteristics of a signal by reducing sampling intervals where the signal changes extremely, and by increasing intervals where the signal changes slowly. Such is disclosed in “Enhancement of Angiograms via M-Band Wavelet Transform” Proceedings of SPIE Biomedical Photonics and Optoelectronic Imaging, 165-169, 2000 written by Yang Yan and Zhang Dong.
Together with Fourier transform, wavelet transform is a method for managing signals which has been actively studied in the 1990s. According to wavelet transform, signals are analyzed by reducing the window size on the time axis for signals with high frequency and increasing the window size on the time axis for signals with low frequency. Therefore, not only information on frequency but also information on time for each band can be known by using the wavelet transform. Also, it is possible to deal with only the signal of a desired band because signals can be divided into bands.
DCT transform is a transformation method transforming data at a two-dimensional area into a two-dimensional frequency plane. The two-dimensional signal obtained as a result of the DCT transform remains the same size and is only expressed in real numbers. Also, since most image signals are concentrated at the low frequency domain, most coefficients which are not 0 are located at the low frequency band, and the coefficients of high frequency elements mostly have a value of 0 or smaller. Therefore, by taking only the meaningful coefficients of the low frequency band and IDCT transforming them, image signals having high frequency elements removed from their image can be obtained. Accordingly, the compression efficiency can be remarkably increased by performing compression using low frequency band coefficients after DCT transforming them, than when performing compression using the coefficient of each pixel of the spatial domain.